Why can't I install Unity as administrator?

I have one account on my PC (Windows Vista OS). It’s an administrator account, doesn’t have any password either and so forth. Whenever I try to install Unity3D it wants me to log onto my administrator account. I can’t use the “Current user” function, and well, again, this is the only account on the computer and it doesn’t have any password, so I can’t type anything into the admin box.

I know this might not be an issue related to Unity3d, but I haven’t encountered this problem with any other program before. Anyone have any solutions?

right click the install, press “Run as Administrator”.

If not, look here: Enable Hidden Admin on Vista